The present invention relates to a transfer device having an endless transfer carrier for use in electrophotographic image formation processing performed by such apparatus as a copying machine, printer and facsimile for example, as well as an image forming apparatus incorporating the transfer device.
Transfer devices of the charger type have been commonly used in electrophotographic image forming apparatus in view of their simplified structures. Such a transfer device produces ozone during electric discharge for obtaining a transfer output, which raises a problem of unpleasant odor and harm to humans' health. For this reason, transfer devices of the contact type have become predominant in these years.
Such contact-type transfer devices are classified into two types, one type being adapted to bring a transfer electrode, such as an electrically conductive roller or brush, into direct contact with the reverse side of a recording sheet to transfer a toner image formed on an image carrier to the recording sheet, the other type being adapted to achieve transfer with use of a transfer carrier, such as an electrically conductive endless belt or film, which intervenes between the image carrier and the recording sheet.
In a transfer device of the latter type using the transfer carrier intervening between the image carrier and the recording sheet, the transfer carrier plays the role of electrostatically attracting a recording sheet thereto for an image to be transferred from the image carrier to the recording sheet and then transporting the recording sheet to fixing means while peeling the recording sheet from the image carrier by keeping the recording sheet electrostatically attracted thereto.
The transfer carrier is applied with transfer bias having a reverse polarity relative to that of charged toner for electrostatically attracting the recording sheet and transferring the image formed on the image carrier. As a result, toner adhering to the image carrier that has not been transferred to the recording sheet is undesirably attracted to the transfer carrier and hence soils the reverse side of a fresh recording sheet to be electrostatically attracted subsequently.
To prevent such an inconvenience, cleaning means is provided for removing toner from the surface of the transfer carrier. Generally-used cleaning means include: means operative to scrape toner adhering to the transfer carrier by contacting the transfer carrier and collect the toner into a collecting container, as disclosed in Japanese Patent No. 3452287 for example; and cleaning means operative to return toner adhering to the transfer carrier to the image carrier by switching transfer bias applied to a transfer bias applying member to cleaning bias having a reversed polarity, as disclosed in Japanese Patent No. 3312800.
The former cleaning means is capable of cleaning the transfer carrier constantly. However, the cleaning means requires the provision of a collecting container for storing collected toner, which leads to upsizing of the apparatus.
The latter cleaning means is incapable of cleaning during image formation. For this reason, in forming an image on multiple recording sheets successively, it is likely that the cleaning means permits toner to soil the reverse side of each recording sheet undesirably.
Further, since the surface potential at a portion of the image carrier directly contacted by the transfer carrier applied with the transfer bias is substantially lower than that at a portion of the image carrier indirectly contacted by the transfer carrier via a recording sheet, a larger amount of toner adheres the transfer carrier via the image carrier, thus soiling an end portion of the reverse side of the recording sheet seriously.
In attempt to solve this problem, there has been provided a transfer device including an auxiliary cleaning member operative to temporarily contact the transfer carrier to hold toner, wherein the auxiliary cleaning member is caused to attract toner adhering to the transfer carrier during successive image formation and then return the attracted toner to the image carrier via the transfer carrier upon completion of image formation on a predetermined number of recording sheets, as disclosed in Japanese Patent No. 3386265.
With this transfer device, however, it is possible that the amount of toner to be attracted by the auxiliary cleaning member exceeds the toner attracting and holding capacity of the auxiliary cleaning member in the case where the size of recording sheets used is relatively large, where the transfer device incorporates a transfer bias controller or the like for controlling the transfer bias depending on the type of recording sheets used, or where a toner replenishing operation is performed, or in a like case. This results in a problem that the auxiliary cleaning member becomes incapable of completely attracting all the toner adhering to the transfer carrier.
A feature of the present invention is to provide a transfer device capable of returning toner to the image carrier via the transfer carrier with controlled timing after having caused an auxiliary cleaning member to attract toner adhering to the transfer carrier thereto and temporarily hold the toner thereon thereby cleaning the transfer carrier reliably, as well as an image forming apparatus incorporating the same.